29 June 2017Monique Robinson, University of Western Australia0 Comment

Will a female pregnancy make mum more emotional, and a male one more aggressive? (iStock)

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This story by The Conversation is republished as part of our series of articles written by local and international academics and researchers who are experts in their field. The views expressed don't necessarily reflect that of Parent24 or Media24.

With advances in prenatal testing it’s now possible to find out whether a pregnancy will result in a male or female baby as early as eight weeks’ gestation.

A quick check of web pregnancy forums would indicate that wait is too long for many parents-to-be who are excited to predict the sex of their baby by interpreting symptoms of pregnancy that might be related specifically to boys or girls.

The old wives’ tales and myths abound when it comes to predicting gender in pregnancy.

Hormone differences for baby boys and girls

There is evidence hormone concentrations in pregnancy can differ according to the sex of the foetus as early as three weeks after fertilisation.

Studies have shown hCG (human chorionic gonadotropin, the hormone responsible for the second line appearing on a home pregnancy test) is higher for female foetuses compared with males, and remains higher throughout pregnancy.

Researchers have found that severe morning sickness (called hyperemesis gravidarum) is slightly more common in pregnancies where the baby is a girl, which could reflect the differences in hCG.

There are plenty of other hormones at work in pregnancy, whatever the sex of the foetus.

There’s a lay belief that when pregnant with a girl, maternal oestrogen levels are higher, but this is not backed up by most studies.

Actually, maternal blood oestrogen levels rise steadily throughout pregnancy whatever the sex of the foetus, although female foetuses do show a higher oestrogen concentration in the amniotic fluid early in the second trimester.

Testosterone in the maternal bloodstream follows a similar pattern, increasing slowly throughout pregnancy with no fetal gender-related differences.

Once again, within the amniotic fluid there are differing concentrations of testosterone though, higher for males than females.

Evidence doesn’t support higher levels of the male or female hormone in the mother’s bloodstream depending on the sex of the foetus. (iStock)

Differences in maternal oestrogen levels in maternal blood have been reported though.

In one Scandinavian study, oestrogen levels in the first half of pregnancy were around 9% higher for pregnancies where the baby was female, and progesterone levels were lower in the second trimester.

But the extent to which the maternal hormone levels matches up to the fetal hormones is difficult to predict.

The foetus is influenced by its own internal hormones in addition to being exposed to circulating maternal hormones.

Getting samples of fetal blood is complicated and risky, so there is much we don’t know about this relationship. There is great interest in this type of research beyond parents who’d like to get the nursery painted early.

The researchers posited that disgust is a protective mechanism that helps the mother avoid risks and harms during pregnancy - for example, aversions to certain foods or situations.

They found that disgust was high in the first trimester for pregnancies bearing boys and girls, but it decreased for girl pregnancies after the first trimester and remained high for boy pregnancies into the second trimester.

The reasoning for this is that male foetuses are considered more vulnerable to their environment, and the aversions reflect a natural predisposition to maintain protective behaviours longer for male pregnancies than female pregnancies.

So it appears there’s not enough evidence to back up claims male or female pregnancies differ significantly in terms of the maternal hormonal environment.

This makes it unlikely that anecdotes of moodier, angrier or uglier pregnancies are due to the sex of the foetus.